Preheat the oven to 200°C, fan 180°C, gas 6.
Whizz half the parsley with the thyme and garlic in a food processor until finely chopped. Put the herbs on a plate and roll the pork in them to coat well.
Heat a large, non-stick, ovenproof frying pan over a medium-high heat and add half the oil. When hot, add the tenderloin and sear the meat on all sides for 5 minutes, then transfer to the oven and cook for 25 minutes. Rest the pork for 10 minutes before slicing thinly.
Meanwhile, heat the remaining oil in a frying pan and lightly fry the celery with the mustard seeds. Mix in the beans and heat through for 2 minutes.
Finely chop the remaining parsley. Mix with the vinaigrette in a bowl and pour 50ml into the beans. Serve the dressed beans with the pork and drizzle over more vinaigrette, as desired.